1. Field of the Invention
The present invention relates to a method of molding a heat-resistant container, and in particular to a method of molding a heat-resistant container from a synthetic resin such as polyethylene terephthalate (PET).
2. Description of the Related Art
Generally, a thin-walled packaging container of synthetic resin known as a biaxially stretch blow molded container is obtained by forming a preform by injection molding or extrusion, and disposing the preform in a blow mold, then stretching the preform longitudinally with a stretching rod while stretching the preform laterally by means of the pressure of a gas blown into the preform.
However, depending on the resin material used to form the container, if the container is filled with heated contents, such as a heat-sterilized fruit juice drink, there is a heat-resistance problem in that the container may be deformed. For example, a PET container formed by conventional blow molding as described above does not have heat resistance.
Various methods have been proposed for imparting heat resistance to a PET container.
For example, in Japanese Patent Application Laid-Open No. 8-187768 (corresponding to U.S. patent application No. 08/544,544) by the assignee of the present application is proposed a method of molding a heat-resistant container.
This method of molding a heat-resistant container comprises the following three steps. First, the preform is subjected to primary blow molding, to form a primary blow-molded article whose length in the long axis is longer than that of the finished product. Next, in a heat treatment mold having a cavity surface substantially of the same shape of the outer surface of the primary blow-molded article, the primary blow-molded article is subjected to heat treatment at a temperature (150 to 220.degree. C.) to promote crystallization of the PET resin. Finally, the intermediate article removed from the heat treatment mold and in the softened and shrunken state is blow molded in a final blow mold having a cavity surface whose shape is the same as that of the finished product, thus forming a final product with heat resistance.
According to this method of molding a heat-resistant container, a container with excellent heat resistance can be formed, and among many other benefits, the required heat treatment can be carried out in a short time, and the shape and size of the shrunken intermediate article after heat treatment is stable.
The above method, by using a heat treatment mold, yields vastly better production efficiency, since the heating efficiency is higher than with the conventional technique of using hot air. However, compared with the manufacturing cycle for containers for carbonated drinks and the like which do not require heat resistance, a long time is required for heat treatment in the heat treatment mold of the above method, and further improvements in production efficiency are sought.
Moreover, as a result of the fall in temperature of the intermediate article in the interval after the heat treatment step and until the final blow molding step, the final molded product may not be precisely blow-molded according to design in the final blow molding, and there is the problem of a loss of dimensional accuracy. As factors in this temperature drop may be cited the removal of air from within the primary blow-molded article after the heat treatment is completed, and the transport of the intermediate article from the heat treatment step to the final blow molding step.
The heat treatment mold used in the second, heat treatment, step comprises a neck support member (for example a neck guide member) supporting the neck portion of the primary blow-molded article, a cavity mold coming into contact with the body of the primary blow-molded article, and a bottom mold coming into contact with the bottom of the primary blow-molded article. Here, while the cavity mold is heated to between 150 and 220.degree. C., the crystallization temperature, the neck support member and bottom mold are maintained at a lower temperature (30 to 100.degree. C.). For this reason, the temperature of the cavity mold tends to fall below the temperature of crystallization in the two borderline regions adjacent to the neck support member and to the bottom mold because of the heat flow in those regions.
These two regions correspond to a first portion extending from the lower part of the neck to the shoulder of the primary blow-molded article and a second portion extending from the base to the heel of the primary blow-molded article, and there is a danger that crystallization will be inadequate in these regions. As a result, the heat resistance of these two portions tends to be inferior to the heat resistance of other portions.
Moreover, these first and second portions are thicker than other parts of the container, and thus harder to heat, and further have a low degree of orientation from the primary blow molding step, as a result of which there is a tendency for the heat resistance to be further impaired by comparison with other portions.